Conventionally, battery-powered controller devices, such as a remote controller and a wireless keyboard, that establish wireless communication with a device to be controlled have been in wide use to facilitate frequent and remote accesses to the controlled device by the user and to free the user from constraints on mobility and location.
The battery mounted in the controller device, needless to say, should be replaced or charged as it wears out and fails to power the controller device. Therefore, desirably, the controller device is equipped with a capability to display a warning to urge the user to replace or charge the battery when the battery has worn out and needs to be replaced or charged, or preferably, before the battery has worn out but needs to be replaced or charged shortly.
Taking Japanese Laid-Open Patent Application No. 6-153268/1994 (Tokukaihei 6-153268; published on May 31, 1994), Japanese Laid-Open Patent Application No. 6-245281/1994 (Tokukaihei 6-245281; published on Sep. 2, 1994), and Japanese Laid-Open Patent Application No. 9-331585/1997 (Tokukaihei 9-331585; published on Dec. 22, 1997) disclosing battery life display apparatuses as examples of conventional technology, as shown in FIG. 7, if the user operates a remote controller 103 (that is, if "YES" is chosen in S101b in FIG. 8), in the remote controller 103, a voltage detection circuit 141 composed of an AD converter and other components converts the divisional voltage value of the output voltage of a battery 131 from analogue to digital so that a microcomputer 143 detects the remaining power of the battery 131 according to the converted value (S102b). Further, the microcomputer 143 causes a transmission section 144 to transmit a battery voltage code denoting the result of the detection, as well as a key operation code, to a system main section 105 (S103b).
Meanwhile, in the system main section 105, if a receiving section 161 receives a signal (if "YES" is chosen in S101a), a microcomputer 162 judges the remaining power of the battery 131 based on the received signal to carry out a display on a source voltage display section 151.
Steps S101b through S103b and S101a through S102a are repeated for every key input; the remote controller 103 detects the voltage of the battery 131 and transmits a battery voltage code in response to every key input.
In the conventional battery voltage display apparatus, the detection of the battery voltage and the transmission of the result of the detection are repeated for every key operation. This increases power consumption by the remote controller, and decreases operational time of the whole wireless communication system.
Specifically, composed of analogue circuits, such as a differential amplifier and an AD converter, the voltage detection circuit 141 typically monitors an output voltage of the battery 131 to detect a drop in the voltage exceeding a certain value. Moreover, one of the analogue circuits is a constant current circuit provided in an amplifier, and another is a resistance division circuit used for generation of a reference voltage. A steady current is flowing through these circuits. The voltage detection circuit 141 is also powered by the battery 131; the steady power consumption by the voltage detection circuit 141 wears out the battery 131. The voltage detection circuit 141 consumes electric power supplied by the battery 131 also when it detects the output voltage of the battery 131; if the more often the detection is conducted, the more quickly the battery 131 wears out.
Moreover, as the result of the detection is transmitted from the transmission section 144 to the system main section 105, the transmission section 144 consumes more electric power than the electric power consumed during reception and the electric power consumed by the microcomputer 143. Therefore, the battery 131 quickly wears out if the result of the detection of the battery voltage is transmitted often, for example, in response to every key operation.
In addition, the battery voltage is continuously monitored with the result being transmitted, even when the system main section 105 does not need detection of the battery voltage any longer, for example, after the battery voltage has dropped below a specified voltage level and the system main section 105 has displayed a warning notifying that the battery should be replaced. This operation causes the battery 131, which is already running low in power supply, to wear out more quickly.
Here, in the battery-powered wireless communication system, the battery-powered device, such as a remote controller or a wireless keyboard, has restrictions on its dimensions and weight so as to offer better portability; the battery 131 is thus allowed only to have a small power supply capacity. Therefore, there is a great demand for a battery voltage display apparatus that can detect the remaining power of the battery on a low power consumption when incorporated in a battery-powered device.